1. Field of the Invention
This invention relates, generally, to methods for removing concrete from the inside of ready mixed concrete truck drums. More particularly, it relates to methods that do not require a worker to enter into the drum, thereby protecting the worker from various occupational hazards.
2. Description of the Prior Art
Ready mixed concrete drums are rotatably mounted on concrete trucks so that the concrete in the drum can be continuously mixed, typically with the drum rotating in a clockwise direction, as it is transported from a concrete batching facility to a job site. Upstanding helical fins or blades are mounted on the interior walls of the rotating drum so that concrete at the closed end of the drum is driven to the open end of the drum when the drum is rotated in a counterclockwise, discharge direction. The helical fins or blades act as an auger, urging the concrete towards the trailing end of the drum when the drum is in said discharge mode. The helical fins or blades urge the concrete toward the cab of the truck when the drum is rotating in the clockwise, mixing direction.
Some ready mixed concrete trucks are front-discharging. The mixing and discharging modes are reversed relative to rear-discharge trucks.
It is inevitable that some concrete will remain within the drum after each load of concrete has been discharged. Over time, the drum is laden with residual concrete that gradually builds up, substantially increasing the weight of the truck when empty and substantially reducing the volume of concrete that the truck can legally haul. The residual concrete also adversely affects the quality of the ready mixed concrete carried by the truck. Some companies combat this problem by attempting to clean the drum at the end of each work day, before the build-up becomes severe. Others just wait until the problem becomes acute.
The conventional way to clean hardened concrete out of a ready mixed concrete drum is to position a worker inside the drum. The worker operates a pneumatic chipping hammer to break the concrete into chips that can be removed. The shortcomings of this well-known procedure are many—the entry into the work space is confined and therefore requires confined space entry permitting, the worker may experience eye injuries, tripping or slipping and falling, and exposure to silica and other harmful particles as the concrete is chipped. Moreover, the worker's hearing is adversely affected in view of the small size of the confined space where the pneumatic chipping hammer is operated, the worker may damage the truck drum and the helical fins or blades when breaking through a chunk of concrete, and so on. Moreover, such workers are paid by the weight of the concrete that they remove. This fact, coupled with the fact that the small workspace is claustrophobic, results in the worker typically leaving small, relatively light pieces of hardened concrete behind. These small pieces act like concrete magnets when new concrete is charged into the drum—they quickly bond with the new concrete, grow rapidly in size and the cycle begins again, forcing another inefficient pneumatic hammer cleaning.
Several inventors have addressed the problem. U.S. Pat. Nos. 6,418,948 and 6,640,817 to Harmon disclose an elongate wand having a plurality of nozzles at its leading end. The nozzles are aimed so that they cause water under pressure to impinge upon the back surface of the helical fins or blades as the wand is inserted into the drum. No cleaning takes place when the wand is retracted from the drum because the flow of water stops when the leading end of the wand contacts the forward, closed end of the drum. The wand does not clean the front side of the helical fins or blades. Moreover, the wand is positioned on the axis of rotation of the drum so that the nozzle is close to the concrete only at the opposite ends of the drum, i. e., at the closed leading end and the open, discharge end. The Harmon wand is positioned in coincidence with the rotational axis of the drum so that it does not come into contact with the helical fins or blades.
The Harmon nozzles are several feet from the residual concrete at the center of the drum because the diameter of the drum is greatest at its center. The efficiency of the cleaning drops off sharply as the distance between the nozzles and the concrete, known in the industry as the stand-off distance, is increased. The nozzles are therefore least effective at the center of the drum because the stand-off distance is greatest at said center.
Two other patents that disclose means for directing high pressure water against the back surface of the helical fins or blades are U.S. Pat. No. 5,244,498 to Steinke and Swedish Patent No. 8802328-8 to Sverige.
Multiple nozzles are not as effective as a single nozzle for cutting concrete. What is needed, then, is a single, oscillating nozzle that cleans concrete from the sidewalls of the drum between the fins or blades, and also from both sides of the fins or blades, not just the back side.
There is also a need for a nozzle that remains at an effective stand-off distance to the concrete to be removed at all times.
The helical fins or blades represent an obstacle to fulfillment of such need. A more specific need exists, accordingly, for a nozzle that remains at a predetermined, highly efficient stand-off distance from the hardened concrete despite the helical fins or blades so that residual concrete is attacked with greater energy.
Instead of finding a way to position a nozzle close to the residual concrete during the cleaning process, the prior art positions the nozzle along the longitudinal axis of symmetry of the drum at all times and uses high water pressure in an ineffective attempt to blast off residual concrete from a relatively long distance.
What is needed is a system that cleans the drum thoroughly, not leaving behind small pieces of concrete that act as magnets or seeds for rapid residual concrete accumulation. The needed system should clean both sides of the helical fins or blades as well, and should do so with the lowest flow rate and water pressure required so as to conserve resources.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified needs could be fulfilled.